New requirements for the working frequency range of vibration sensors are constantly raised with the development of science and technology. Vibration sensors with a lower-limit working frequency lower than 0.05 Hz began to be widely adopted in the fields of aeronautics and astronautics, weapon guidance, earthquake prediction, ultra-precision manufacturing, mineral exploration, etc. The appearance of these sensors sets higher requirement for ultralow frequency measurement and calibration systems.
According to the national metrological verification regulations of China (JJG 134-2003, JJG 233-2008), the vibration sensor should be calibrated by the metrological administrative department before delivery or after being used for a period of time (usually every one year). Vibration sensors are usually calibrated through a standard vibration device, which generates standard horizontal or vertical sinusoidal vibration in the calibration system. The primary calibration and secondary calibration methods are used to calibrate the sensor to get its various indicators. The accuracy of the output waveform of the standard exciter determines the calibration accuracy of the sensor.
The amplitude of the output velocity, acceleration signal of the vibration exciter is small and the signal-to-noise ratio is low at a low frequency and especially at an ultralow frequency. In addition, under the condition of equal voltage input, the exciters exhibit displacement characteristics at the low frequency. Generally, the displacement sensor is frequently adopted as the measurement and feedback control device. However, common displacement sensor has low precision and special contact-type displacement sensor with high precision would cause friction and tension to the moving component of exciter. Such traditional displacement sensor cannot be used in the control system of metrological standard vibration exciter.